Date and Time

Where do you go when you want to calculate birth date from date of death and exact age, to the day, when someone died? I first checked out the calendar tool in Family Tree Maker, but from the description it wasn't clear how it handled the change in the calendar from the Julian to the Gregorian system. The British Empire changed to the Gregorian calendar in 1752 when 11 days were skipped 2 September 1772 being followed by 14 September. There is a good Wikipedia article on the Gregorian calendar here.
Fortunately there is a calculator at dateandtime.com which makes if very clear the calendar change is being included. Find it here.
Did the person who provided the exact age information you're using do the calculation properly?